Several examples of these articles are known, such as catheters having a friction-reducing coating. In connection with catheters, the presence of the friction-reducing coating causes the surface of the catheter to be slippery and lubricating when the catheter is dipped into an aqueous, optionally saline solution prior to the insertion into a body cavity or when the catheter is contacted with an aqueous body liquid at the insertion into the body cavity. Thus, the discomfort experienced by the patient when the catheter is inserted into and removed from the body cavity is considerably reduced. The risk of damaging sensitive tissue in connection with the use of the catheter is at the same time considerably reduced.
The literature discloses several examples of coatings of the above type, in which the binder is based on polyurethane and the hydrophilic polymer is based on poly-(N-vinyllactam), cf. for instance GB-PS No. 1,600,963 (Micklus et al.), U.S. Pat. No. 4,666,437 (Lambert), U.S. Pat. No. 4,642,267 (Creasy et al.), and WO publication No. 90/05162 (Uno Plast A/S). In addition, Danish printed accepted application No. 159,018 (Lambert) discloses coatings based on an interpolymer of hydrophilic polyethyleneoxide and hydrophobic polyurea.
The friction-reducing coatings based on a hydrophilic coating have a much lower coefficient of friction in a wet condition than in a dry condition. It turned out, however, that such hydrophilic coatings may have a tendency to dry out, with the result that the article is insufficiently hydrophilic and that the coefficient of friction is increased.
Thus, tests on hydrophilic polymer coatings on various substrates have shown that when an article coated with a hydrophilic polymer coating is dipped into water, the coating is well wetted, but a high risk exists of the polymer coating losing its water content when contacted with the mucosa or the like due to the difference in the osmotic potential between the hydrophilic coating and the mucosa. The mucosa has a higher osmotic potential, i.e., a higher concentration of salt than the hydrophilic coating, with the result that the water leaves the hydrophilic layer and enters the mucosa, whereby the difference in the concentrations of salt is equalized. It is obvious that such a drying out involving an increased coefficient of friction can be highly unpleasant, such as when the catheter is to be removed.
A solution of this problem is described in U.S. Pat. No. 4,906,237 (corresponding to EP-PS No. 217,771 and Danish Pat. Application No. 4532/86; Johansson et al.), in which it is suggested to treat the polymer coating with a solution containing at least 2% salt, mono or disaccharide or a sugar alcohol, whereafter the solvent is evaporated from the solution. Such a treatment ensures the presence of an osmolality-increasing compound in the hydrophilic coating, whereby the above difference in the osmotic potentials in the coating and the surrounding mucosa is avoided. The suggested treatment with a solution followed by evaporation of the solvent, usually water, is, however, a time-consuming process with respect to production because the evaporation of water is a difficult and time-consuming process step for several reasons. Thus, water is per se a solvent being relatively difficult to volatilize, having a high boiling point, and requiring an extensive supply of evaporation heat. In addition, the added osmolality-increasing compound involves lowering of the vapour pressure, and the volatility of the water is accordingly additionally reduced. Finally, the hydrophilic coating binds and retains the water in the coating. During the step of the drying process, in which only a minor amount of water is left in the coating, the coating becomes adhesive, which involves further difficulties concerning the handling of the coated article which, of course, must be completely dry and non-adhering at the end of the process.
The articles coated by the method of Johansson et al. are furthermore encumbered with the drawback that the crystallization or the precipitation of the osmolality-increasing compound cannot be controlled, and the crystallization or precipitation is accordingly dictated by the conditions prevailing during the drying process after the application of the compound (c) in the dissolved condition. During the crystallization or the precipitation, the process is thus subject to arbitrary conditions caused by an irregular crystallization speed or precipitation speed and a non-uniform diffusion of the dissolved compound during the application performed by way of dipping for a long period, such as one hour.
Thus, a demand exists for a method of producing articles with coatings of the above type where the method is less complicated and less time-consuming with respect to the productional technique, and where the resulting coatings maintain a low friction when the article has been inserted into a body cavity, and in which the osmolality-increasing compound is more uniformly dispersed in the coating.